The invention relates to an X-ray pulse source for generating X-ray pulses, in particular to an X-ray pulse source including an electromagnetic radiation based undulator. Furthermore, the invention relates to a method of creating X-ray pulses, in particular comprising the steps of photo-induced generating electron pulses, accelerating the electron pulses and creating the X-ray pulses by an interaction of the electron pulses and electromagnetic pulses. Applications of the invention are available in creating X-rays e. g. for imaging and/or investigation purposes, in particular for attosecond imaging and spectroscopy, for seeding an X-ray Free Electron Laser, for phase contrast imaging, X-ray crystallography and spectroscopy, lithography, X-ray scattering techniques, or ultrafast X-ray analysis.
For describing the background of the invention, particular reference is made to the following publications:    [1] G. A. Krafft et al. in “Physical Review” vol. E 72, no. 5 (2005): 056502;    [2] E. Esarey et al. in “Physical Review” vol. E 48, no. 4 (1993): 3003;    [3] F. C. Jones et al. in “Physical Review” vol. 167, no. 5 (1968): 1159;    [4] K. Ta. Phuoc et al. in “Nature photonics” vol. 6, no. 5(2012): 308-311;    [5] N. D. Powers et al. in “Nature Photonics” vol. 8, no. 1 (2014): 28-31;    [6] S. Chen et al. in “Physical Review Letters” 110, no. 15 (2013): 155003;    [7] Gil Travish et al. “An Optical-Scale Period Undulator for Hard X-ray Production from Compact Devices?” FLS 2012, March 5-9 at Thomas Jefferson National Accelerator Facility;    [8] Gil Travish et al. in “SPIE Optics+ Optoelectronics. International Society for Optics and Photonics,” 2011.    [9] C. Maroli, V. Petrillo, L. Serafini, A. Bacci, A. Rossi, and P. Tomassini, “Compact X-ray Free Electron Laser Bases on an Optical Undulator,” Proceedings of FEL 2007, Novosibirsk, Russia;    [10] L. Serafini et al. in “Particle Accelerator Conference, 2001” Proceedings of the PAC 2001. 2001, vol. 3, pp. 2242-2244. IEEE, 2001;    [11] F. Grüner et al. in “Applied Physics” B 86, no. 3 (2007): 431-435;    [12] T. M. Tran et al. in “Quantum Electronics, IEEE Journal of” 23, no. 9 (1987): 1578-1589.    [13] C. Chang et al. in “Physical Review letters” 110, no. 6 (2013):064802;    [14] J. Gea-Banacloche et al. in “Nuclear Instruments and Methods in Physics Research” Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 272, no. 1 (1988): 199-205;    [15] J. Gea-Banacloche et al. in “Quantum Electronics, IEEE Journal of ” 23, no. 9 (1987): 1558-1570;    [16] J. C. Gallardo et al. in “Quantum Electronics, IEEE Journal of” 24, no. 8 (1988):1557-1566;    [17] A. Bacci et al. in “Physical Review—Special Topics—Accelerators and Beams” 9, no. 6 (2006): 060704;    [18] W. S. Graves et al. in “Physical Review Letters” 108, no. 26 (2012): 263904;    [19] W. R. Huang et al. “A terahertz-driven electron gun” arXiv preprint arXiv:1409.8668 (2014).    [20] L. J. Wong et al. in “Optics Express” vol. 21, no. 8 (2013): 9792-9806;    [21] E. A. Nanni et al. in “International Particle Accelerator Conference”, Dresden 2014, WEOAB03;    [22] R. B. Yoder et al. in “Physical Review—Special Topics Accelerators and Beams” vol. 8, no. 11 (2005): 111301;    [23] S. Tantawi et al. in “Physical Review Letters” vol. 112, no. 16 (2014): 164802;    [24] P. Hommelhoff et al. in “Physical Review Letters” vol. 96, no. 7 (2006): 077401;    [25] R. Bormann et al. in “Physical Review Letters” vol. 105, no. 14 (2010): 147601;    [26] P. Dombi et al. in “Nano Letters” vol. 13, no. 2 (2013): 674-678;    [27] B. Piglosiewicz et al. in “Nature Photonics” vol. 8, no. (2014): 37-42;    [28] H. Yanagisawa et al. in “Physical Review Letters” vol. 107, no. 8 (2011):087601;    [29] P. D. Keathley et al. in “Annalen der Physik” vol. 525, no. 1-2 (2013): 144-150;    [30] M. Kruger et al. in “Nature” vol. 475, no. 7354 (2011): 78-81;    [31] A. Mustonen et al. in “Applied Physics Letters” vol. 99, no. 10 (2011): 103504;    [32] M. E. Swanwick et al. in “Nano Letters” vol. 14, no. 9 (2014): 5035-5043;    [33] R. G. Hobbs et al. in “ACS nano” 8 (11), pp 11474-11482 (2014);    [34] S.-H. Huang et al. in “Opt. Lett.” 38:(5), 796-798 (2013);    [35] U.S. Pat. No. 7,382,861;    [36] US 2014/0314114;    [37] US 2012/0288065; and    [38] U.S. Pat. No. 5,150,192.
X-rays are the most powerful tool to understand structure and function of materials from the micro scale down to the atomic level. The structure of every virtual material in our daily lives has been determined by X-rays and some of the most powerful medical imaging tools are based on X-ray technology. X-ray sources with multi keV photon energy are needed to perform a variety of applications ranging e. g. from medical imaging, like phase contrast imaging in the X-ray domain to X-ray crystallography. Practical requirements to the X-ray sources comprise in particular compactness, coherence and brilliance.
Compact coherent X-ray sources are based on high-order harmonic generation (HHG), which is so far the only technique to generate completely, i.e. spatially and temporally, coherent X-rays. Electrons are ionized from an atom in a gas via a strong field of a femtosecond laser. Some of these electrons ionized with the correct phase with respect to the driving laser field are accelerated within approximately a half-cycle and re-collide with the partly ionized atom which drives a strong polarization wave in the atom-returning electron system that leads to the emission of sub-femtosecond or attosecond pulses of EUV to soft-X-ray photons.
Limitations of the HHG technique result from the relatively low photon energies (hard X-ray range is not available) and low brilliance for high photon energies. In addition, achieving a coherent X-ray radiation from an HHG process is challenging and still under debate.
Alternatively, available X-ray sources with the desired beam properties and brilliance are based on relativistic electron beams from linear accelerators, i.e., large scale Free Electron Lasers (FEL). Facilities like LCLS in the US and the future European XFEL project are linear accelerator (LINAC) based and accelerate electrons to highly relativistic (10 GeV) energies. This high energy is necessary for an undulator with a typical period of 3 cm and total length of 100 m to produce coherent radiation via the self-amplified stimulated emission process (SASE) in the FEL. A LINAC relies on room temperature or super-conducting RF technology. The accelerating gradients in either case are limited to several tens of MeV per meter limited by field emission from cavity walls. The LINAC length, therefore, must be in the km regime and facility costs are in the billion Euro category.
The concept of colliding a relativistic electron bunch with an electromagnetic wave to produce X-ray radiation can be considered in two main regimes, namely incoherent radiation and coherent radiation. In the incoherent regime (so-called inverse Compton scattering or Thomson scattering, [1]-[6]), the electrons in a bunch move within a counter-propagating electromagnetic wave and consequently radiate an electromagnetic wave. The produced radiation is completely incoherent due to the random distribution of the electrons.
The coherent regime uses sources in which the radiation of electrons affects the electron bunch and makes the electrons ordered at the radiation wavelength scale. This leads then to coherent radiation or the so-called FEL regime. In this case, one usually considers the counter-propagating electromagnetic wave as an optical or in general electromagnetic undulator.
The use of electromagnetic undulators for the production of coherent X-ray beams has been widely presented over the last decades. Conventionally, the relativistic electrons which collide with an optical beam are produced according to one of the following approaches. In [7] and [8], the electrons are accelerated using a high power laser beam coupled into a dielectric nanostructured device. However, due to the small wavelengths of the optical lasers, the accelerated bunches using the considered laser acceleration scenario, are too small to radiate sufficiently in the X-ray regime. In [9], radiofrequency (RF) linear accelerators are considered to inject electrons into an optical undulator. There are also several proposals for a compact X-ray source based on electron bunches from laser plasma acceleration [11].
Furthermore, using a general electromagnetic wave undulator without focusing on the type of electron accelerators has been described in [14], [15], [17] and [35]. In [36], THz beams have been used as an undulator. Similarly, using a microwave undulator combined with RF accelerated electron beams is experimentally reported in [23]. Various studies focus on the efficiency of the source and properties of the X-ray radiation when different electromagnetic undulator types, namely a standing wave [12] or a Gaussian beam [16], are used.
The common point between all these prior art ideas is the use of optical or THz undulatory, which is mainly motivated by the aimed compactness of the overall device. Various schemes for electron acceleration are pursued in the previous proposals, namely radio frequency (RF) acceleration, optical acceleration in dielectric waveguides and laser-plasma wake field acceleration. However, RF accelerators require high RF-power sources and large facilities due to the long wavelength and need to fill large cavity volumes and low accelerating gradients, making the ultimate device not compact. Optical acceleration in dielectric waveguides is a solution recently studied to achieve short accelerated electron bunches. The main drawback of this scheme is the strong limits it puts on the bunch charge, or simply the number of electrons in a single bunch that can be accelerated. Since the coherent X-ray emission is proportional to the square of the number of electrons in a bunch, not much emission can be expected from this approach. Additionally, the accelerated electron bunch suffers from a large energy spread. Both disadvantages emanate from the short wavelength of the electromagnetic wave. The third strategy, which is the laser-plasma wakefield acceleration, has the potential for a compact device, low emittance beams and also high charge electron bunches. In fact, due to these outstanding capabilities, this method has received extensive attention in the last years. Nonetheless, the scheme suffers from large energy spreads in the produced relativistic electrons due to plasma instabilities and the robustness of bunch parameters from shot to shot.
Another approach to achieve a coherent X-ray beam without benefiting from the undulator radiation is introduced in [18] and [37], where a periodic arrangement of electrons emitted from nano-emitter arrays is sent to an emittance exchange line. Subsequently, the longitudinally ordered electrons will radiate coherently when scattered off an electromagnetic wave.
Few publications have focused on THz acceleration for producing relativistic electrons. In [19], the acceleration of electrons using a THz beam is presented. In [20] and [22], THz acceleration in waveguides is theoretically studied and recently performed experimentally [21]. However, the concept of using THz accelerated and compressed electrons was restricted to theoretical considerations.
The use of optically driven field emitter arrays to produce electron beams for injection into an accelerator has been studied widely in the last years. These devices make use of enhanced optical fields at sharp metal tips. The high fields allow for operation in the strong-field emission regime, where ultrafast response times are possible and consequently short bunches can be produced ([24]-[30]). Ultrafast field emitter arrays [38] have been studied previously both theoretically and experimentally ([31]-[33]).
In brief summary, FEL sources are the only devices capable of producing high brightness, spatially and temporally coherent hard X-ray radiation. However, the current large scale facilities require high energy and charge electron beams, which does not allow for a compact format and the utilized RF-technology makes them inappropriate for sub-fs pulse generation. The high cost of FEL based X-ray sources does not lead to a widespread use in X-ray research and limits the speed of progress in this area. Therefore, there is a strong interest in a compact (laboratory scale) source, even if incoherent but providing critical flux levels with much higher spatial coherence compared to today's laboratory sources. Such a source could lead to a widespread dissemination, like conventional optical lasers, into research labs, materials characterization laboratories and hospitals.